Study: 'social chromosome' identified in fire ants

Geneticists studying fire ants have published a paper detailing the discovery of what they are calling the
first known social chromosome -- an inherited group of genes, which
together dictate behaviour.

The find came about during an investigation into the group
behaviour of the species Solenopsis invicta. The
species has two distinct social colony structures stemming from
which type of queen ant heads it up, explains Elizabeth Norton in
ScienceNow. On the one hand there is the large,
fertile and rather dominant monogyne queen -- a colony bred from
this queen will be highly territorial and aggressive, killing any
new queen that attempts to attach itself to the colony. A polygyne
colony, as the name suggests, will accept multiple queens. It is
primarily headed up by a smaller queen who doesn't mind getting a
helping hand bolstering the numbers of her brood.

For decades biologists have tried to uncover the genetic
reasoning behind such different behaviours being displayed within
one species, behaviours which go deep into explaining the colony
structure as a whole. While monogyne ants appear inherently
aggressive and are threatened by intruders, a polygyne colony is
predisposed to accept strangers and only gets its guard up if a
monogyne queen infringes on it (in which case, she is
slaughtered).

Thanks to genome sequencing, we finally have an answer.

In the hunt to identify the gene or group of genes responsible for
dictating social behaviour in fire ants, the team -- headed up by
Laurent
Keller of Lausanne University -- had already
sequenced the fire ant genome. Differences in monogyne and polygyne
ants' Gp-9 gene had already been identified in the 90s, so this is
where their investigation focused. After studying the DNA of 500
specimens it became clear that the ants' behavioural differences
are built into their DNA in a similar way to how gender is.

Ordinarily, genetic information from two parents is muddled to
make new and unique genetic encoding. This process is referred to
as recombination. However, when it comes to sex chromosomes, one
chunk of genetic information located in the Y-chromosome is unable
to recombine. The Y-chromosome has evolved to retain the genetic information that
ensures important male characteristics are never muddled in a gene
exchange. Keller and his team found that a similar suppression of
the recombination practice is happening in fire ant DNA, around
where the Gp-9 gene is located. It's located within a pair of
heteromorphic chromosomes (two chromosomes that share some
characteristics but are not identical), with the two possible
versions referred to as social B (SB) and social b (Sb). They found
that 55 percent of the chromosome is unable to recombine, with a
total of 616 genes retained and inherited. SB chromosomes can
exchange information between each other, but genetic information
cannot be crossed between SB and Sb, as in sex chromosomes.
Therefore, as the paper states, "locally limiting recombination" is
responsible for the inherited traits.

Further bolstering this argument, the authors concluded:
"importantly, most of the genes with demonstrated expression
differences between individuals of the two social forms reside in
the non-recombining region."

The group suggests this suppression is down to "at least one large
inversion of around 9 megabases" -- meaning a strand of DNA has
been inverted and thus could be preventing chunks of genes from
detaching and reattaching as they normally would during
recombination.

Speaking about the find to ScienceNow, Kenneth Ross -- who identified the disparate
Gp-9 gene in monogyne and polygyne fire ants back in the 90s
-- called it "a spectacular piece of work".

"They've unlocked a whole new mechanism for how a
supergene can determine something as complex as
behaviour."